1. Field of the Invention
The present invention relates to a photo receiver used in an optical transmission system and more particularly to a preamplifier capable of preventing an output waveform from distorting when a large light is input.
The present application claims the priority of Japanese Patent Application No. Hei11-303170 filed on Oct. 25, 1999, which is hereby incorporated by reference.
2. Description of the Related Art
In an optical transmission system, in order to convert a received input light into a signal voltage and in order to output the signal voltage, a circuit is generally used in which the received input light is converted into a current via a photodiode and then is amplified by a preamplifier so that a required signal voltage output is obtained.
A conventional preamplifier used for this purpose is disclosed in Japanese Patent Application Laid-open No. Hei 4-306904.
In the conventional preamplifier, as shown in FIG. 16, a photodiode 104 is connected between an input terminal 102 and a bias source V.sub.H of a differential amplifier 101, a reference voltage V.sub.ref is connected to a reference input terminal 107, a feedback resistor R.sub.f is connected between an inverting output terminal 103 and the input terminal 102, an FET (Field Effect Transistor) Q100 is connected in parallel with the feedback resistor R.sub.f, a level converting circuit 106 is connected to a positive phase output terminal 105 and an output of the level converting circuit 106 is connected to a gate of the FET Q100.
In operation of the conventional preamplifier, when an input light is received, an optical current I.sub.ph flows into the input terminal 102 via the photodiode 104. With this operation, an output voltage V.sub.out occurs at the inverting output terminal 103. At this time, feedback is applied via the feedback resistor R.sub.f, and thereby a constant amplifying gain can be obtained. When the input light level is excessive, an output occurs from the level converting circuit 106 caused by the output voltage from the positive phase output terminal 105, therefore, the FET Q100 becomes conductive and a feedback resistance value becomes small. As a result, the amplifying gain lowers and the output voltage V.sub.out is prevented from increasing, therefore, the differential amplifier 101 is prevented from saturating.
Also, there is disclosed an another conventional preamplifier used for the above-mentioned purpose is disclosed in Japanese Patent Application Laid-open No. Hei 4-306905.
In the another conventional preamplifier, as shown in FIG. 17, a photodiode 104 is connected between an input terminal 102 and a bias source V.sub.H of a differential amplifier 101, a reference voltage V.sub.ref is connected to a reference input terminal 107, a feedback resistor R.sub.f is connected between an inverting output terminal 103 and the input terminal 102, an FET (Field Effect Transistor) Q101 is connected between the input terminal 102 and a ground potential V.sub.BB, a level converting circuit 106 is connected to a positive phase output terminal 105 and an output of the level converting circuit 106 is connected to a gate of the FET Q101.
In operation of the another conventional preamplifier, when an input light is received, an optical current I.sub.ph flows into the input terminal 102 via the photodiode 104. With this operation, an output voltage V.sub.out occurs at the inverting output terminal 103. At the time, a feedback is applied via the feedback resistor R.sub.f, and thereby a constant amplifying gain can be obtained. When the input light level is excessive, an output occurs from the level converting circuit 106 caused by the output voltage from the positive phase output terminal 105, therefore, the FET Q101 becomes conductive and the optical current I.sub.ph is distributed to the ground potential V.sub.BB. As a result, amplifying gain lowers and the output voltage V.sub.out is prevented from increasing, therefore, the differential amplifier 101 is prevented from saturating.
With the conventional preamplifiers shown in FIG. 16 and FIG. 17, when the input light exceeds a predetermined level, the output voltage V.sub.out is prevented from increasing, therefore, the differential amplifier 101 can be prevented from saturating.
However, at this time, since the level converting circuit 106 detects increment in a polarity direction of a positive phase output voltage and then generates an output, one side of a waveform of the output voltage V.sub.out is clipped in accordance with an operation of the FET Q100 or the FET Q101.
FIG. 18 shows an example of the output voltage when an input current is bypassed in the conventional preamplifiers shown in FIG. 16 and FIG. 17.
In FIG. 18, a .vertline.voltage.vertline. designates a potential difference of both ends of the feedback resistor R.sub.f after bypassing an over-current, a V.sub.0 designates a voltage when an optical current I.sub.ph is minimum, an I.sub.ph XR.sub.f designated by a dotted line designates a voltage when the optical current I.sub.ph is maximum and the differential amplifier 101 is not saturated. Actually, when the optical current I.sub.ph is large, the output voltage is limited in accordance with the operation of the FET Q100 or the Q101. As a result, therefore, the .vertline.voltage.vertline. varies asymmetrically up and down as designated by a solid line.
As above described, in the conventional preamplifier circuit, there is a problem in that an output waveform is distorted when the optical input level is excessive.